Constant switching frequency DTC for induction motor fed from two level voltage source inverter

Direct torque control (DTC) of an induction motor fed by a two-level voltage source inverter (VSI) is a simple scheme that need less computation time, can be implemented without mechanical speed sensors and is insensitive to parameter variations. In principle, the motor terminal voltages and currents are sampled and used to estimate the motor flux and torque. Based on estimates of the flux position and the instantaneous errors in torque and stator flux magnitude, a voltage vector is selected to restrict the torque and the flux errors within their respective torque and flux hysteresis bands. DTC based on hysteresis controllers suffer from certain drawbacks such as variable switching frequency and high torque ripple. This paper proposes a constant switching frequency DTC based on the concept of imaginary switching time. The proposed algorithm is simple to implement and avoids the requirement of reference voltage vector, sector identification and angle determination. This paper presents simulation study to compare the performance of the proposed DTC with conventional DTC. Simulation is carried out in MATLAB/Simulink. Simulation results clearly show that, there is significant reduction in the torque ripple while the proposed DTC is used and the locus of the flux is nearly a circle. The proposed scheme is implemented using TMS320F2812 DSP evaluation board.